1. Field of the Invention
The present invention relates to a quartz-crystal unit, and more particularly, to a crystal unit which is suitable for applications that require heat resistance.
2. Description of the Related Art
It is widely known that a quartz-crystal oscillator using a quartz-crystal unit provides a stable oscillation frequency. The influence exerted by a change in temperature is the largest factor which contributes to fluctuations in the oscillation frequency of the crystal oscillator. For example, a crystal oscillator using an AT cut crystal unit, which is most often employed, presents a change in frequency in a range of several tens to several hundreds of ppm to a change in temperature from xe2x88x9230xc2x0 C. to +80xc2x0 C. For this reason, a crystal oscillator for use in applications which require a stable frequency source such as measuring instruments, base stations of a mobile communication system, and the like is an oven-controlled type one which accommodates a crystal unit in a thermostatic oven. The oven-controlled type crystal oscillator prevents a change in oscillation frequency due to a change in temperature by accommodating the crystal unit in the thermostatic oven which is heated at a constant temperature of approximately +80xc2x0 C. In this way, the crystal unit for use in the oven-controlled type crystal oscillator is used in a high temperature environment at approximately +80xc2x0 C.
On the other hand, a crystal unit used at temperatures near a room temperature typically has excitation electrodes formed on both surfaces of a quartz crystal blank and extended to edges of their respective surfaces.
Then, the crystal blank is held at extreme ends of the extended excitation electrodes by holding members, each of which comprise a wire, a thin metal plate, or the like formed with a clip at a leading end thereof. A conductive adhesive is further applied on the sites at which the crystal blank is held to ensure secure fixation of the crystal blank and to make electric conduction between the holding members and excitation electrodes. The conductive adhesive for use in this case may be, for example, a mixture of an adhesive based on epoxy resin and thin pieces, grains, or the like of silver.
However, when a crystal unit using a conductive adhesive is placed in a high temperature environment, a gas component generated from organic components of the adhesive, in particular, sticks on the excitation electrodes, causing a change over time in the resonant frequency of the crystal unit, i.e., the oscillation frequency of a crystal oscillator over time. Therefore, if a crystal unit for use in room temperatures as described above is used in a oven-controlled type crystal oscillator, the crystal unit of which is exposed to high temperatures of approximately +80xc2x0 C., the oscillation frequency will change over time.
To solve this problem, a structure as illustrated in FIG. 1 is contemplated for a crystal unit for use in a thermostatic oven. The crystal unit illustrated in FIG. 1 has two terminals 22 extending through base 21, and holding member 23 attached at the leading end of each terminal 22.
On the other hand, both side ends of disk-shaped crystal blank 24 are applied with a fillet comprised of a mixture of a low-melting glass and a silver filler, which is heated to a temperature exceeding 400xc2x0 C., at which the fillet is molten, to form connections 26. Since the low-melting glass is chemically similar to quartz in components, it can firmly secure and form connections 26 at predetermined positions of crystal blank 24. Then, excitation electrodes 25 are formed by vapor deposition at the centers of top and bottom faces of crystal blank 24, opposite to each other. Excitation electrodes 25 are extended in directions opposite to each other to positions spaced by a predetermined distance from connections 26 at the ends of the surfaces.
Then, connections 26 of crystal blank 24 are held by holding members 23, and a brazing material composed of gold and germanium (Auxe2x80x94Ge), used to form a eutectic alloy, is heated to approximately 350xc2x0 C. to bond holding members 23 to connections 26.
When bonding by the gold-germanium brazing material is performed under the condition that excitation electrodes 25 directly contact with holding members 23, excitation electrodes 25 are eroded by electrolytic etching, disadvantageously causing degeneration of excitation electrodes 25 and resulting gradual change in the resonant frequency. In extreme cases, crystal blank 24 could come off holding members 23. However, when connections 26 mainly composed of silver are formed such that spacings are defined between connections 26 and excitation electrodes 25, connections 26 can be securely bonded to holding members 23 using the gold-germanium brazing material without adversely affecting excitation electrodes 25.
Then, metal thin films 27 are vapor deposited on the top and bottom faces of crystal blank 24 to cover the extended ends of excitation electrodes 25 and connections 26, thereby providing electric conduction between excitation electrodes 25 and connections 26.
Subsequently, a trace of metal film is additionally vapor deposited on excitation electrodes 25 to finely adjust the resonant frequency of crystal blank 24 to a target frequency by its mass addition effect.
Then, cover 28 having an open lower end is fitted over base 21, and the opening end is bonded to a flange along the peripheral edge of base 21 by soldering, cold pressure welding, or the like, with the internal space of cover 28 placed in an inert gas or vacuum atmosphere, to hermetically encapsulate crystal blank 24.
The conventional heat resistant crystal unit is assembled in the foregoing manner. In the conventional crystal unit, crystal blank 24 is held in a direction perpendicular to base 21.
In recent years, however, surface mount devices tend to be more often used in a variety of electric devices for purposes of automated assembling processes, reduction in size, and the like. The surface mount type is also required for the crystal unit. A surface mount crystal unit employs, for example, a container which has a base made of ceramic. The base has outer shape in a rectangular parallelepiped and is formed with a recess on the top face. With this container, after a crystal blank is accommodated in the recess, a metal-made lid is seam welded along the opening of the recess to encapsulate the crystal blank. Therefore, in such a surface mount crystal unit, the crystal blank is accommodated in the recess of the base in parallel with the bottom face thereof. Thus, if connections of the crystal blank are secured to holding members formed on the bottom face of the recess with a gold-germanium brazing material for forming a heat resistant crystal unit of the structure described above, excitation electrodes below the crystal blank cannot be applied with vapor deposition and the like. This results in a problem that the underlying excitation electrode cannot be electrically connected to the holding member.
Also, some crystal units of a general type having lead lines, not for surface mounting, can hold a crystal blank horizontally on a holding member disposed on a base. Such a crystal unit is similar in that a lower surface of the crystal blank cannot be applied with vapor deposition and the like when the crystal blank is held by the holding member. In this structure, therefore, underlying excitation electrodes cannot either be electrically connected to the holding member.
It is an object of the present invention to provide a crystal unit which has good heat resistance, wherein a crystal blank is held horizontally to a base and securely fixed to holding members with a gold-germanium brazing material, and excitation electrodes on the top and bottom faces of the crystal blank are connected to the holding members by vapor deposition.
The object of the present invention is achieved by a crystal unit which includes a base, a pair of holding members attached to the base, a crystal blank formed in a plate shape for exciting piezoelectric vibrations and having a first and a second surface, a first and a second connection each formed on the second surface by applying a fillet including a mixture of a low-melting glass and a metal filler, and melting the fillet by heating, a third connection formed at an end of the crystal blank to extend over both of the first and second surfaces by applying a fillet including a low-melting glass and a metal filler, and melting the fillet by heating, a first excitation electrode formed on the first surface and having a first lead electrically conducting to the third connection, a second excitation electrode formed on the second surface corresponding to the first excitation electrode and having a second lead arranged a predetermined spacing apart from the first connection, a first metal thin film for making electric conduction between the second lead and the first connection, and a second metal thin film for making electric conduction between the second connection and the third connection. The first and second connections are brazed to the pair of holding members, respectively.
According to the present invention, the crystal unit provided thereby holds the surfaces of the crystal blank in parallel with the base, and exhibits high heat resistance which makes it suitable for use in a thermostat oven heated at a constant temperature.